1. Field of the Invention
The present invention relates to a coating apparatus applicable as a paper coater in a paper making machine.
2. Description of the Prior Art
FIG. 3 shows one example of a coating machine (short dwell coater) in the prior art, in which a web (of paper) 3 is adapted to be transported as wrapped around an outer circumference of a backing roller 2. The backing roller 2 includes a drum and elastic material such as rubber or the like wrapped around the outer circumferential surface of the drum. A coating apparatus is provided at a lower portion of the aforementioned backing roller 2. The apparatus is provided with a blow-off section 5 for injecting the coating liquid vertically upward towards a lower surface of the web of paper 3, and a blade 7 integral with the blow-off section 5 at the downstream side of the apparatus with respect to the paper web transport direction. The blade 7 is provided for removing surplus coating liquid to form a film of coating liquid having a constant thickness on the web of paper 3. In addition, for the purpose of damming up the coating liquid within a chamber 4, a dam plate 8 is disposed at the upstream side of the apparatus with respect to the paper web transport direction.
With regard to the short dwell coater shown in FIG. 3, some of the coating liquid injected through opening 9 is transferred onto the web 3, the remainder of the coating liquid which has reached the blade 7 as a result of the transportation of the same web becomes surplus liquid and is mixed with the newly supplied coating liquid, and the mixed coating liquid circulates within the chamber 4 as indicated at 12. Part of the circulating coating liquid does not flow out beyond the dam plate 8.
In such a coating apparatus, when coating liquid having a large amount of hydrophobic pigment is employed, when employing coating liquid containing a binder in a small proportion, or when the web speed is high (800 m/min. or higher) and when the ability of the web to absorb water has been lowered by pretreatment, the thickness of the coating liquid becomes uneven in the widthwise direction of the web even if metering is effected by means of the blade 7. Hence, the thickness of a film on the final product would be also adversely influenced. That is, when the coated paper web is looked through, or is observed under a fluorescent exciting lamp if the coating liquid contains fluorescent dye, in some cases a belt-like unevenness of the film is observed.
The coating liquid in the prior art as shown in FIG. 9 would circulate within the chamber, owing to the upward pressure and the scraping of the coating liquid by the blade 7. However, the circulating flow is not uniform along the widthwise direction of the web, especially when the machine operates at a high speed. And in some cases, the liquid layer coated on the web would not have an even thickness.